The present invention relates generally to apparatus for testing tires and more particularly to a control means for controlling the load on a tire held in loaded engagement with a rotating test road wheel.
It is well-known in the art to test tires for operating and endurance characteristics by holding them in a loaded condition against a rotating road wheel for a specified length of time and at desired conditions of load, inflation pressure, speed and ambient temperature. For a considerable number of years tire manufacturing companies have performed these tests to gather information for their own tire evaluation purposes. In addition, in the past few years, the United States Department of Transportation (D.O.T.) has also required that these tests be made under specified conditions and that results of such tests be supplied to the D.O.T. A critical requirement in such tests is that the specified load be maintained throughout the test period which frequently lasts a number of days of continuous running. Older type road wheel testing machines utilized a manual application of weights which, through leverage arms or other suitable arrangements, applied load to the tire undergoing testing. However, such machines required continuous attention from test personnel due to the lack of a self-regulating load control. Moreover, since a tire exhibits characteristics generally associated with springs, the load actually applied to the tire would vary even though the weight on the test machine was constant. More recently, load control mechanisms have been developed in which tire load feedback information is utilized to operate a servo system in which the load is continuously adjusted by a motor driven screw or hydraulic pump to maintain it constant on the tire. Again, because of the spring-like characteristics of a tire, the servo system oscillates substantially about the preset load point. This is particularly a problem in a screw type loading system which has a slow reaction time and therefore hunts to a greater degree about the set load point. Further, in hydraulic type servo systems, the null bias present in all servo valves also causes the control to continuously oscillate about the reference load set point. The consequence of the continuous oscillating of such feedback servo systems is that the hydraulic pump and valve or screw motor and related controls are in a continuous operating state and have a relatively short useable life. In addition, because of the continuous operation of such controls, it is difficult to detect the inception of tire failure prior to the time the tire disintegrates by monitoring the operation of the controls.